Subject and structure
The subject line decides whether the email gets opened. It needs to say what it is about using words from the conversation: "Freight quote for the upstate routes" works; "Follow-up" or "Contact" say nothing.
In the body, four parts: one line reminding where you spoke, one paragraph with what you understood about the need, the request and a signature with name, company and phone. If you attach a proposal or material, say in the text what it is and why it matters.